Added application session services, along with initial restore and tracker implementations.

Harmonia.WinUI/Session/ISessionRestorer.cs

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+namespace Harmonia.WinUI.Session;
+
+public interface ISessionRestorer
+{
+    void Restore();
+}

Harmonia.WinUI/Session/ISessionService.cs

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+using System.Threading.Tasks;
+
+namespace Harmonia.WinUI.Session;
+
+public interface ISessionService
+{
+    ApplicationSession Session { get; }
+    Task LoadAsync();
+    Task SaveAsync();
+}

Harmonia.WinUI/Session/ISessionTracker.cs

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+namespace Harmonia.WinUI.Session;
+
+public interface ISessionTracker
+{
+    void Start();
+}

Harmonia.WinUI/Session/JsonSessionService.cs

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+using System;
+using System.IO;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+namespace Harmonia.WinUI.Session;
+
+public sealed class JsonSessionService : ISessionService
+{
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        WriteIndented = true
+    };
+
+    private readonly string _filePath = Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
+        "Harmonia",
+        "session.json"
+    );
+
+    public ApplicationSession Session { get; private set; } = new();
+
+    public async Task LoadAsync()
+    {
+        try
+        {
+            if (File.Exists(_filePath))
+            {
+                await using var stream = File.OpenRead(_filePath);
+                Session = await JsonSerializer.DeserializeAsync<ApplicationSession>(stream) ?? new();
+            }
+        }
+        catch
+        {
+            Session = new();
+        }
+    }
+
+    public async Task SaveAsync()
+    {
+        string directoryName = Path.GetDirectoryName(_filePath)!;
+        Directory.CreateDirectory(directoryName);
+
+        await using var stream = File.Create(_filePath);
+        await JsonSerializer.SerializeAsync(stream, Session, Options);
+    }
+}

Harmonia.WinUI/Session/SessionRestorer.cs

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+using Harmonia.Core.Player;
+using Harmonia.Core.Playlists;
+using System;
+using System.Linq;
+
+namespace Harmonia.WinUI.Session;
+
+public class SessionRestorer(
+    ISessionService sessionService,
+    IPlaylistManager playlistManager,
+    IAudioPlayer audioPlayer) : ISessionRestorer
+{
+    public void Restore()
+    {
+        RestorePlaylist();
+        RestoreAudioPlayer();
+    }
+
+    private void RestorePlaylist()
+    {
+        PlaylistState playlistState = sessionService.Session.Playlist;
+
+        if (string.IsNullOrWhiteSpace(playlistState.OpenPlaylistUID))
+            return;
+
+        Playlist? playlist = playlistManager.Playlists.FirstOrDefault(p => p.UID == playlistState.OpenPlaylistUID);
+
+        if (playlist is null)
+            return;
+
+        playlistManager.CurrentPlaylist = playlist;
+    }
+
+    private void RestoreAudioPlayer()
+    {
+        AudioPlayerState audioPlayerState = sessionService.Session.Player;
+
+        if (string.IsNullOrWhiteSpace(audioPlayerState.PlaylistSongUID))
+            return;
+
+        Playlist? playlist = playlistManager.FindPlaylistContaining(audioPlayerState.PlaylistSongUID);
+
+        if (playlist is null)
+            return;
+
+        PlaylistSong? song = playlist.Songs.FirstOrDefault(s => s.UID == audioPlayerState.PlaylistSongUID);
+
+        if (song is null)
+            return;
+
+        //PlaybackMode playbackMode = audioPlayerState.PlaybackState
+
+        audioPlayer.LoadAsync(song, PlaybackMode.LoadOnly);
+    }
+}

Harmonia.WinUI/Session/SessionTracker.cs

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+using Harmonia.Core.Player;
+using Harmonia.Core.Playlists;
+using System;
+
+namespace Harmonia.WinUI.Session;
+
+public class SessionTracker(
+    ISessionService sessionService,
+    IPlaylistManager playlistManager,
+    IAudioPlayer audioPlayer) : ISessionTracker
+{
+    public void Start()
+    {
+        playlistManager.CurrentPlaylistChanged += OnCurrentPlaylistChanged;
+        audioPlayer.PlayingSongChanged += OnPlayingSongChanged;
+    }
+
+    private void OnCurrentPlaylistChanged(object? sender, EventArgs e)
+    {
+        PlaylistState playlistState = sessionService.Session.Playlist;
+        playlistState.OpenPlaylistUID = playlistManager.CurrentPlaylist?.UID;
+    }
+
+    private void OnPlayingSongChanged(object? sender, PlayingSongChangedEventArgs e)
+    {
+        AudioPlayerState audioPlayerState = sessionService.Session.Player;
+        audioPlayerState.PlaylistSongUID = e.NewSong?.UID;
+    }
+}